SUMMARY Project 2 Bile acids have long been known to be critical for efficient intestinal absorption of fats and lipid-soluble vitamins. Recent studies reveal that bile acids have widespread metabolic effects, and are associated with key components of the metabolic syndrome (MetSyn), including glucose/insulin homeostasis, plasma lipid levels, and maintenance of gut microbiota. Bile acid homeostasis is maintained via complex regulatory mechanisms, with many components still being incompletely understood. A fascinating aspect of bile acid homeostasis is the coordination between bile acid uptake in intestine and the control of bile acid synthesis in liver. We recently used a genetic approach to identify the Diet1 gene, and determined that Diet1 acts as a control point for the intestinal production of fibroblast growth factor 15/19 (FGF15/19), which signals in liver to repress bile acid synthesis. Mice with a Diet1 null mutation have reduced FGF15 secretion, causing impaired feedback repression of hepatic bile acid synthesis, and increased fecal bile acid excretion. As a result, these mice are resistant to hyperlipidemia and atherosclerotic lesions. Due to its recent identification, many aspects of Diet1 function are unknown. In Specific Aim 1, we will further characterize the molecular and physiological role of Diet1 in bile acid homeostasis, glucose homeostasis, and effects on gut microbiome composition. We will also evaluate the effects of DIET1 genetic variants on bile acid levels and MetSyn traits. In Specific Aim 2, we will identify novel genetic loci that determine bile acid levels using the hybrid mouse diversity panel (HMDP), an unparalleled resource for genetic association of loci for complex traits. We have mapped loci for biliary, fecal, and plasma bile acid levels at high resolution to loci containing only 6-16 candidate genes. We will identify the causative genes using a combination of genetic, biochemical, molecular, and in vivo approaches. We will evaluate genes identified in the mouse for association with bile acid levels and MetSyn traits in the METSIM cohort, with 3500 individuals typed for bile acid levels. Our studies are made possible by mouse and human genetic resources that are unique to our PPG, and our findings will contribute to the overall goal of identifying genes that contribute to MetSyn traits.